Co-channel operation systems, methods, and devices

ABSTRACT

Co-channel operation systems, methods, and devices are discussed in this document. Some embodiments can include remote stations configured for co-channel operation with one or more other remote stations. Remote stations can generally comprise a processor and a memory in electronic communication with the processor. Instructions can be stored in the memory, and when executed by the processor cause a remote station to receive a first data sequence from a first base station; use the first data sequence to distinguish a first signal transmitted by the first base station from unwanted signals transmitted by one or more other base stations; and demodulate the first signal. Other aspects, embodiments, and features are also claimed and described.

PRIORITY CLAIM

The present Application for Patent is a continuation of U.S. patentapplication Ser. No. 12/769,778, filed 29 Apr. 2010, now U.S. Pat. No.8,432,824, which claims the benefit of and priority to U.S. ProvisionalApplication No. 61/174,801, filed 1 May 2009. Both of said applicationsare assigned to the assignee hereof and hereby expressly incorporated byreference herein as if fully set forth below in their entireties and forall applicable purposes.

TECHNICAL FIELD

The technology discussed below relates generally to communications andin particular to systems, methods, and devices that relate to aspects ofco-channel operation in a wireless communications system. Someembodiments can be network based while others can be remotecommunication device based.

BACKGROUND

Modern mobile cellular telephones are able to provide conventional voicecalls and data calls. The demand for both types of calls continues toincrease, placing increasing demands on network capacity. Networkoperators address this demand by increasing their capacity. This isachieved, for example, by dividing or adding cells and hence adding morebase stations, which increases hardware costs. It is desirable toincrease network capacity without unduly increasing hardware costs, inparticular to cope with unusually large peak demand during major eventssuch as an international football match or a major festival, in whichmany users or subscribers who are located within a small area wish toaccess the network at one time.

When a first remote station is allocated a channel for communication, asecond remote station can only use the allocated channel after the firstremote station has finished using the channel. Maximum cell capacity isreached when all the allocated channels are used in the cell. This meansthat any additional remote station user will not be able to get service.Co-channel interference (CCI) and adjacent channel interference (ACI)further limit network capacity and will be discussed below.

Network operators have addressed this problem in a number of ways, allof which require added resources and added cost. For example, oneapproach is to divide cells into sectors by using sectored, ordirectional, antenna arrays. Each sector can provide communications fora subset of remote stations within the cell and the interference betweenremote stations in different sectors is less than if the cell were notdivided into sectors. Another approach is to divide cells into smallercells, each new smaller cell having a base station. Both theseapproaches are expensive to implement due to added network equipment. Inaddition, adding cells or dividing cells into smaller cells can resultin remote stations within one cell experiencing more CCI and ACIinterference from neighboring cells because the distance between cellsis reduced.

BRIEF SUMMARY OF SOME SAMPLE EMBODIMENTS

Here is provided a summary of some sample embodiments of the presentinvention. The following summarizes some aspects of the presentdisclosure to provide a basic understanding of the discussed technology.This summary is not an extensive overview of all contemplated featuresof the disclosure, and is intended neither to identify key or criticalelements of all aspects of the disclosure nor to delineate the scope ofany or all aspects of the disclosure. Its sole purpose is to presentsome concepts of one or more aspects of the disclosure in summary formas a prelude to the more detailed description that is presented later.

Some embodiments can include remote stations configured for co-channeloperation with one or more other remote stations. Remote stations cangenerally comprise a processor and a memory in electronic communicationwith the processor. Instructions can be stored in the memory, and whenexecuted by the processor cause a remote station to receive a first datasequence from a first base station; use the first data sequence todistinguish a first signal transmitted by the first base station fromunwanted signals transmitted by one or more other base stations; anddemodulate the first signal.

Other embodiments can include methods that remote stations can employfor co-channel operations with one or more other remote stations. Forexample, a method for co-channel operation by a remote station cancomprise receiving a first data sequence from a first base station;using the first data sequence to distinguish a first signal transmittedby the first base station from unwanted signals transmitted by one ormore other base stations; and demodulating the first signal.

Other aspects, features, and embodiments of the present invention willbecome apparent to those of ordinary skill in the art, upon reviewingthe following description of specific, exemplary embodiments of thepresent invention in conjunction with the accompanying figures. Whilefeatures of the present invention may be discussed relative to certainembodiments and figures below, all embodiments of the present inventioncan include one or more of the advantageous features discussed herein.In other words, while one or more embodiments may be discussed as havingcertain advantageous features, one or more of such features may also beused in accordance with the various embodiments of the inventiondiscussed herein. In similar fashion, while exemplary embodiments may bediscussed below as device, system, or method embodiments it should beunderstood that such exemplary embodiments can be implemented in variousdevices, systems, and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a transmitter and a receiver;

FIG. 2 shows a block diagram of a receiver unit and demodulator of thereceiver shown in FIG. 1;

FIG. 3 shows example frame and burst formats in a TDMA system;

FIG. 4 shows part of a TDMA cellular system;

FIG. 5 shows an example arrangement of time slots for a TDMAcommunications system;

FIG. 6 shows a simplified representation of part of a TDMA cellularsystem adapted to assign the same channel to two remote stations;

FIG. 7 shows example arrangements for data storage within a memorysubsystem which might reside within a base station controller (BSC) of acellular communication system;

FIG. 8 shows a flowchart of a method of assigning a channel already inuse by one remote station to another remote station;

FIG. 9 of the accompanying drawings is a schematic diagram of apparatuswherein the method represented by FIG. 8 resides in a base stationcontroller;

FIG. 10 shows a receiver architecture for a remote station havingenhanced co-channel rejection capability;

FIG. 11 is a schematic diagram of (a) a transmitting apparatus and (b) areceiving apparatus, suitable for selecting a receiving apparatus forco-channel operation;

FIG. 12A is a schematic diagram showing sequences of data frames eachcontaining, or not containing, discovery bursts comprising co-channeldata;

FIG. 12B is a further schematic diagram showing sequences of data frameseach containing, or not containing, discovery bursts comprisingco-channel data.

FIG. 13 is a flow diagram of a method of selecting a receiving apparatusfor co-channel operation;

FIG. 14 is a further flow diagram of a method of selecting a receivingapparatus for co-channel operation;

FIG. 15 is a graph of FER performance under different levels ofsignal-to-noise ratio (Eb/No) for different codecs;

FIG. 16 is a graph of FER performance under different levels of carrierto interference (C/I) for different codecs.

FIG. 17 is a flow diagram of a method of progressively increasing thenumber of discovery bursts within a SACCH period for a series of SACCHperiods.

FIG. 18 of the accompanying drawings shows an apparatus for operating ina multiple access communication system to produce first and secondsignals sharing a single channel.

DETAILED DESCRIPTION

Interference due to other users limits the performance of wirelessnetworks. This interference can take the form of either interferencefrom neighboring cells on the same frequency, known as co-channelinterference (CCI), discussed above, or neighboring frequencies on thesame cell, known as adjacent channel interference (ACI), also discussedabove.

FIG. 1 of the accompanying drawings shows a block diagram of atransmitter 118 and a receiver 150 in a wireless communication system.For the downlink, the transmitter 118 may be part of a base station, andreceiver 150 may be part of a wireless device (remote station). For theuplink, the transmitter 118 may be part of a wireless device such as aremote station, and receiver 150 may be part of a base station. A basestation is generally a fixed station that communicates with the wirelessdevices and may also be referred to as a Node B, an evolved Node B(eNode B), an access point, etc. A wireless device may be stationary ormobile and may also be referred to as a remote station, a mobilestation, a user equipment, a mobile equipment, a terminal, a remotestation, an access terminal, a station, etc. A wireless device may be acellular phone, a personal digital assistant (PDA), a wireless modem, awireless communication device, a handheld device, a subscriber unit, alaptop computer, etc.

At transmitter 118, a transmit (TX) data processor 120 receives andprocesses (e.g., formats, encodes, and interleaves) data and providescoded data. A modulator 130 performs modulation on the coded data andprovides a modulated signal. A transmitter unit (TMTR) 132 conditions(e.g., filters, amplifies, and upconverts) the modulated signal andgenerates an RF modulated signal, which is transmitted via an antenna134.

At receiver 150, an antenna 152 receives the transmitted RF modulatedsignal from transmitter 110 together with transmitted RF modulatedsignals from other transmitters. Antenna 152 provides a received RFsignal to a receiver unit (RCVR) 154. Receiver unit 154 conditions(e.g., filters, amplifies, and downconverts) the received RF signal,digitizes the conditioned signal, and provides samples. A demodulator160 processes the samples and provides demodulated data. A receive (RX)data processor 170 processes (e.g., deinterleaves and decodes) thedemodulated data and provides decoded data. In general, the processingby demodulator 160 and RX data processor 170 is complementary to theprocessing by modulator 130 and TX data processor 120, respectively, attransmitter 110.

In a wireless communications system, the data is multiplexed using amultiplexing technique, so as to allow a plurality of remote stations123-127 (each comprising a receiver 150) to communicate with a singlebase station 110, 111, 114 (comprising a transmitter 118). Examples ofmultiplexing techniques are frequency division multiplex (FDM), and timedivision multiplexing (TDM) or time division multiple access (TDMA). Theconcepts underlying these techniques will be discussed below.

Controllers/processors 140 and 180 control/direct operations attransmitter 118 and receiver 150, respectively. Memories 142 and 182store program codes in the form of computer software, and data used bytransmitter 118 and receiver 150 respectively.

FIG. 2 of the accompanying drawings shows a block diagram of a receiverunit 154 and demodulator 160 of the receiver 150 shown in FIG. 1. Withinreceiver unit 154, a receive chain 440 processes the received RF signaland provides I and Q baseband signals, which are denoted as I_(bb) andQ_(bb). Receive chain 440 may perform low noise amplification, analogfiltering, quadrature downconversion, etc. An analog-to-digitalconverter (ADC) 442 digitalizes the I and Q baseband signals at asampling rate of f_(adc) and provides I and Q samples, which are denotedas I_(adc) and Q_(adc). In general, the ADC sampling rate f_(adc) may berelated to the symbol rate f_(sym) by any integer or non-integer factor.

Within demodulator 160, a pre-processor 420 performs pre-processing onthe I and Q samples from ADC 442. For example, pre-processor 420 mayremove direct current (DC) offset, remove frequency offset, applyautomatic gain control (AGC) etc. An input filter 422 filters thesamples from pre-processor 420 based on a particular frequency responseand provides input I and Q samples, which are denoted as I_(in) andQ_(in) to data filter 422. Data filter 422 may filter the I and Qsamples to suppress images resulting from the sampling by ADC 442 aswell as jammers. Filter 422 may also perform sample rate conversion,e.g., from 24× oversampling down to 2× oversampling. A data filter 424filters the input I and Q samples from input filter 422 based on anotherfrequency response and provides output I and Q samples, which aredenoted as I_(out) and Q_(out). Filters 422 and 424 may be implementedwith finite impulse response (FIR) filters, infinite impulse response(IIR) filters, or filters of other types. The frequency responses offilters 422 and 424 may be selected to achieve good performance. In oneexample, the frequency response of filter 422 is fixed, and thefrequency response of filter 424 is configurable.

An adjacent channel interference (ACI) detector 430 receives the input Iand Q samples from filter 422, detects for ACI in the received RFsignal, and provides an ACI indicator signal to filter 424. The ACIindicator signal may indicate whether or not ACI is present and, ifpresent, whether the ACI is due to the higher RF channel centered at+200 KHz and/or the lower RF channel centered at −200 KHz. The frequencyresponse of filter 424 may be adjusted based on the ACI indicator toachieve good performance.

An equalizer/detector 426 receives the output I and Q samples fromfilter 424 and performs equalization, matched filtering, detection,and/or other processing on the samples. For example, equalizer/detector426 may implement a maximum likelihood sequence estimator (MLSE) thatdetermines a sequence of symbols that is most likely to have beentransmitted given a sequence of I and Q samples and a channel estimate.

In a TDMA system, each base station 110, 111, 114 is assigned one ormore channel frequencies and each channel frequency may be used bydifferent users during different intervals of time known as time slots.For example each carrier frequency is assigned eight time slots (whichare labeled as time slots 0 through 7) such that eight consecutive timeslots form one TDMA frame. A physical channel comprises one channelfrequency and one time slot within a TDMA frame. Each active wirelessdevice/user is assigned one or more time slot indices for the durationof a call. For example during a voice call, a user is allocated one timeslot (hence one channel) at any instant. User-specific data for eachwireless device is sent in the time slot(s) assigned to that wirelessdevice and in TDMA data frames used for the traffic channels.

FIG. 3 of the accompanying drawings shows example frame and burstformats in a TDMA system. In a TDMA system, each time slot within aframe is used for transmitting a “burst” of data. Sometimes the termstime slot and burst may be used interchangeably. Each burst includes twotail fields, two data fields, a training sequence (or midamble) field,and a guard period (labeled GP in the figure). The number of symbols ineach field is shown inside the parentheses in FIG. 3. A burst includes148 symbols for the tail, data, and midamble fields. No symbols are sentin the guard period. TDMA frames of a particular carrier frequency arenumbered and formed in groups of 26 or 51 TDMA frames calledmultiframes.

For traffic channels used to send user-specific data, each multiframe inthis example includes 26 TDMA frames, which are labeled as TDMA frames 0through 25. The traffic channels are sent in TDMA frames 0 to 11 and inTDMA frames 13 to 24 of each multiframe. A control channel is sent inTDMA frame 12. No data is sent in idle TDMA frame 25, which is used bythe wireless devices to make measurements for neighbor base stations110, 111, 114.

FIG. 4 of the accompanying drawings shows part of a TDMA cellular system100. The system comprises base stations 110, 111 and 114 and remotestations 123, 124, 125, 126 and 127. Base station controllers 141 to 144act to route signals to and from the different remote stations 123-127,under the control of mobile switching centres 151, 152. The mobileswitching centres 151, 152 are connected to a public switched telephonenetwork (PSTN) 162. Although remote stations 123-127 are commonlyhandheld mobile devices, many fixed wireless devices and wirelessdevices capable of handling data also fall under the general title ofremote station 123-127.

Signals carrying, for example, voice data are transferred between eachof the remote stations 123-127 and other remote stations 123-127 bymeans of the base station controllers 141-144 under the control of themobile switching centres 151, 152. Alternatively, signals carrying, forexample, voice data are transferred between each of the remote stations123-127 and other communications equipment of other communicationsnetworks via the public switched telephone network 162. The publicswitched telephone network 162 allows calls to be routed between themobile cellular system 100 and other communication systems. Such othersystems include other mobile cellular communications systems 100 ofdifferent types and conforming to different standards.

Each of remote stations 123-127 can be serviced by any one of a numberof base stations 110, 111, 114. A remote station 124 receives both asignal transmitted by the serving base station 114 and signalstransmitted by nearby non-serving base stations 110, 111 and intended toserve other remote stations 125.

The strengths of the different signals from base stations 110, 111, 114are periodically measured by the remote station 124 and reported to BSC144, 114, etc. If the signal from a nearby base station 110, 111 becomesstronger than that of the serving base station 114, then the mobileswitching centre (MSC) 152 acts to make the nearby base station 110, 111become the serving base station and acts to make the serving basestation 114 become a non-serving base station. The MSC 152 thus performsa handover of the remote station to the nearby base station 110.Handover refers to the method of transferring a data session or anongoing call from one channel to another.

In cellular mobile communications systems, radio resources are dividedinto a number of channels. Each active connection (for example a voicecall) is allocated a particular channel having a particular channelfrequency for the downlink signal (transmitted by the base station 110,111, 114 to a remote station 123-127 and received by the remote station123-127) and the channel having a particular channel frequency for theuplink signal (transmitted by the remote station 123-127 to the basestation 110, 111, 114 and received by the base station 110, 111, 114).The frequencies for downlink and uplink signals are often different, toallow simultaneous transmission and reception and to reduce interferencebetween transmitted signals and received signals at either the remotestation 123-127 or the base station 110, 111, 114. This is known asfrequency division duplex (FDD).

FIG. 5 of the accompanying drawings shows an example arrangement of timeslots for a TDMA communications system. A base station 114 transmitsdata signals in a sequence of numbered time slots 30, each signal beingfor only one of a set of remote stations 123-127 and each signal beingreceived at the antenna of all remote stations 123-127 within range ofthe transmitted signals. The base station 114 transmits all the signalsusing time slots on an allocated channel frequency. Each channelfrequency and time slot combination thus comprises a channel forcommunication. For example, a first remote station 124 and a secondremote station 126 are both allocated the same channel frequency. Thefirst remote station 124 is allocated a first time slot 3 and a secondremote station 126 is allocated a second time slot 5. The base station114 transmits, in this example, a signal for the first remote station124 during time slot 3 of the sequence of time slots 30, and transmits asignal for the second remote station 126 during time slot 5 of thesequence of time slots 30.

The first and second remote stations 124, 126 are active during theirrespective time slots 3 and 5 of time slot sequence 30, to receive thesignals from the base station 114. The remote stations 124, 126 transmitsignals to the base station 114 during corresponding time slots 3 and 5of time slot sequence 31 on the uplink. It can be seen that the timeslots for the base station 114 to transmit (and the remote stations 124,126 to receive) 30 are offset in time with respect to the time slots forthe remote stations 124, 126 to transmit (and the base station 114 toreceive) 31.

This offsetting in time of transmit and receive time slots is known astime division duplexing (TDD), which among other things, allows transmitand receive operations to occur at different instances of time.

Voice signals and data signals are not the only signals to betransmitted between the base station 110, 111, 114 and the remotestation 123-127. A control channel is used to transmit data thatcontrols various aspects of the communication between the base station110, 111, 114 and the remote station 123-127. Among other things, thebase station 110, 111, 114 uses the control channel to send to theremote station 123-127 a sequence code, or training sequence code (TSC)which indicates which of a set of sequences the base station 110, 111,114 will use to transmit the signal to the remote station 123-127. InGSM, a 26-bit training sequence is used for equalization. This is aknown sequence which is transmitted in a signal in the middle of everyburst.

The sequences are used by the remote station 123-127: to compensate forchannel degradations which vary quickly with time; to reduceinterference from other sectors or cells; and to synchronize the remotestation's receiver to the received signal. These functions are performedby an equalizer which is part of the remote station's 123-127 receiver.An equalizer 426 determines how the known transmitted training sequencesignal is modified by multipath fading. The equalizer may use thisinformation to extract the desired signal from unwanted reflections ofthe signal by constructing an inverse filter to extract parts of thedesired signal which have been corrupted by multipath fading. Differentsequences (and associated sequence codes) are transmitted by differentbase stations 110, 111, 114 in order to reduce interference betweensequences transmitted by base stations 110, 111, 114 that are close toeach other.

A remote station 123-127 which comprises a receiver having enhancedco-channel rejection capability is able to use the sequence todistinguish the signal transmitted to it by a base station 110, 111, 114from other unwanted signals transmitted by other base stations 110, 111,114. This holds true so long as the received amplitudes or power levelsof the unwanted signals are below a threshold relative to the amplitudeof the wanted signal. The unwanted signals can cause interference to thewanted signal if they have amplitudes above this threshold. Thethreshold can vary according to the capability of the remote station's123-127 receiver. The interfering signal and the desired (or wanted)signal can arrive at the remote station's 123-127 receivercontemporaneously if, for example, the signals from the serving andnon-serving base stations 110, 111, 114 share the same time slot fortransmitting. An example of remote station 123-127 which has enhancedco-channel rejection capability is a remote station 123-127 comprising areceiver having downlink advanced receiver performance (DARP), which isdescribed in cellular standards such as those defining the system knownas Global System for Mobile communication (GSM) which is an example of aTDMA system.

A remote station 123-127 which has enhanced co-channel rejectioncapability by means of DARP, is able to use the training sequences todistinguish a first signal from a second signal and to demodulate anduse the first signal, when the amplitudes of the first and secondsignals are substantially within, say, 10 dB of each other. Each DARPmobile station will treat the signal intended for another mobile station123-127 as co-channel interference (CCI) and reject the interference.

Referring again to FIG. 4, at remote station 124 transmissions from basestation 110 for remote station 125 can interfere with transmissions frombase station 114 for remote station 124. The path of the interferingsignal is shown by dashed arrow 170. Similarly, at remote station 125transmissions from base station 114 for remote station 124 can interferewith transmissions from base station 110 for remote station 125 (thepath of the interfering signal shown by dotted arrow 182).

TABLE 1

Table 1 above shows example values of parameters for signals transmittedby the two base stations 110 and 114 illustrated in FIG. 4. Theinformation in rows 3 and 4 of the table show that for remote station124 both a wanted signal from a first base station 114 and an unwantedinterferer signal from a second base station 110 and intended for remotestation 125 are received and the two received signals have the samechannel and similar power levels (−82 dBm and −81 dBm respectively).Similarly, the information in rows 6 and 7 shows that for remote station125 both a wanted signal from the second base station 110 and anunwanted interferer signal from the first base station 114 and intendedfor remote station 124 are received and the two received signals havethe same channel and similar power levels (−80 dBm and −79 dBmrespectively).

Each remote station 124, 125 thus receives both a wanted signal and anunwanted interferer signal that have similar power levels from differentbase stations 114, 110, on the same channel (i.e. contemporaneously).Because the two signals arrive on the same channel and similar powerlevels, they interfere with each other. This may cause errors indemodulation and decoding of the wanted signal. This interference is theco-channel interference discussed above.

The co-channel interference may be mitigated to a greater extent thanpreviously possible, by the use of DARP-enabled remote stations 123-127,and base stations 110, 111, 114 having enhanced co-channel rejectioncapability. DARP capability may be implemented by means of a methodknown as single antenna interference cancellation (SAIC) or by means ofa method known as dual antenna interference cancellation (DAIC).

The DARP feature works better when the amplitudes of the receivedco-channel signals are similar. This situation may typically occur wheneach of two remote stations 123-127, each communicating with a differentbase station 110, 111, 114, is near a cell boundary, where the pathlosses from each base station 110, 111, 114 to each remote station123-127 are similar.

A remote station 123-127 that is not DARP-capable, by contrast, may onlydemodulate the wanted signal if the unwanted co-channel interferersignal has an amplitude, or power level, lower than the amplitude of thewanted signal. In one example, it must be lower by at least 8 dB inorder to allow the receiver to demodulate the wanted signal. TheDARP-capable remote station 123-127 can therefore tolerate a muchhigher-amplitude co-channel signal relative to the wanted signal, thancan the remote station 123-127 not having DARP capability.

The co-channel interference (CCI) ratio is the ratio between the powerlevels, or amplitudes, of the wanted and unwanted signals expressed indB. In one example, the co-channel interference ratio could be, forexample, −6 dB (whereby the power level of the wanted signal is 6 dBlower than the power level of the co-channel interferer (unwanted)signal). In another example, the ratio may be +6 dB (whereby the powerlevel of the wanted signal is 6 dB higher than the power level of theco-channel interferer (unwanted) signal). For DARP-enabled remotestations 123-127 with good performance, the remote stations 123-127 canstill process the wanted signal when the amplitude of the interferersignal is around 10 dB higher than the amplitude of the wanted signal,and. If the amplitude of the interferer signal is 10 dB higher than theamplitude of the wanted signal, the co-channel interference ratio is −10dB.

DARP capability, as described above, improves a remote station's 123-127reception of signals in the presence of ACI or CCI. A new user, withDARP capability, will better reject the interference coming from anexisting user. The existing user, also with DARP capability, would dothe same and not be impacted by the new user. In one example, DARP workswell with CCI in the range of 0 dB (same level of co-channelinterference for the signals) to −6 dB (co-channel is 6 dB stronger thanthe desired or wanted signal). Thus, two users using the same ARFCN andsame timeslot, but assigned different TSCs, will get good service.

The DARP feature allows two remote stations 124 and 125, if they bothhave the DARP feature enabled, to each receive wanted signals from twobase stations 110 and 114, the wanted signals having similar powerlevels, and each remote station 124, 125 to demodulate its wantedsignal. Thus, the DARP enabled remote stations 124, 125 are both able touse the same channel simultaneously for data or voice.

The feature described above of using a single channel to support twosimultaneous calls from two base stations 110, 111, 114 to two remotestations 123-127 is somewhat limited in its application in the priorart. To use the feature, the two remote stations 124, 125 are withinrange of the two base stations 114, 110 and are each receiving the twosignals at similar power levels. For this condition, typically the tworemote stations 124, 125 would be near the cell boundary, as mentionedabove. It is desirable to increase, by some other means, the number ofactive connections to remote stations that can be handled by a basestation.

A method and apparatus will now be described which allows the supportingof two or more simultaneous calls on the same channel (consisting of atime slot on a carrier frequency), each call comprising communicationbetween a single base station 110, 111, 114 and one of a plurality ofremote stations 123-127 by means of a signal transmitted by the basestation 110, 111, 114 and a signal transmitted by the remote station123-127. This supporting of two or more simultaneous calls on the samechannel is known as Multi-User on One Slot (MUROS) or as Voice servicesover Adaptive Multi-user on One timeSlot (VAMOS). Since two trainingsequences may be used for signals in the same time slot on the samecarrier frequency in the same cell by the same base station 110, 111,114, twice as many communication channels may be used in the cell.

FIG. 6 of the accompanying drawings shows a simplified representation ofpart of a TDMA cellular system adapted to assign the same channel to tworemote stations 125, 127. The system comprises a base station 110, andtwo remote stations 125, 127. The network can assign, via the basestation 110, the same channel frequency and the same time slot (i.e. thesame channel) to the two remote stations 125 and 127. The networkallocates different training sequences to the two remote stations 125and 127 which are both assigned: a channel frequency having frequencychannel number (FCN) equal to 160; and a time slot with time slot index(TS) equal to 3. Remote station 125 is assigned a training sequence code(TSC) of 5 whereas 127 is assigned a training sequence code (TSC) of 0.Each remote station 125, 127 will receive its own signal (shown by solidlines in the figure) together with the co-channel (co-TCH) signalintended for the other remote station 125, 127 (shown by dotted lines inthe figure). Each remote station 125, 127 is able to demodulate its ownsignal whilst rejecting the unwanted signal.

DARP, when used along with the embodiments described herein, thereforeenables a TDMA network to use a channel already in use (i.e., a channelfrequency and time slot that is already in use) to serve additionalusers. In one example, each channel can be used for two users forfull-rate (FR) speech and by four users for half-rate (HR) speech. It isalso possible to serve a third or even a fourth user if the users'receivers have sufficiently good DARP performance. In order to serveadditional users using the same channel, the network transmits theadditional users' RF signals on the same carrier (channel frequency),using optionally different phase shifts, and assigns to the additionalusers the same timeslot that is in use, using a different TSC from thatused by the current user. The transmitted bursts of data each comprisethe training sequence corresponding to the TSC. A DARP capable receivermay detect the wanted or desired signal for that receiver whilerejecting the unwanted signal for another receiver. It is possible toadd third and fourth users in the same way as for the first and secondusers.

Single-antenna interference cancellation (SAIC) is used to reduceCo-Channel Interference (CCI). The 3G Partnership Project (3GPP) hasstandardized SAIC performance. The 3GPP adopted the term ‘downlinkadvanced receiver performance’ (DARP) to describe the receiver thatapplies SAIC.

DARP increases network capacity by employing lower reuse factors.Furthermore, it suppresses interference at the same time. DARP operatesat the baseband part of a receiver of a remote station 123-127. Itsuppresses adjacent-channel and co-channel interference that differ fromgeneral noise. DARP is available in previously defined GSM standards(since Rel-6 in 2004) as a release-independent feature, and is anintegral part of Rel-6 and later specs. The following is a descriptionof two DARP methods.

The first DARP method is the joint detection/demodulation (JD) method.JD uses knowledge of the GSM signal structure in adjacent cells insynchronous mobile networks to demodulate one of several interferencesignals in addition to the desired signal. JD's ability to demodulateinterference signals allows the suppression of specific adjacent-channelinterferers. In addition to demodulating GMSK signals, JD also can beused to demodulate EDGE signals. Blind interferer cancellation (BIC) isanother method used in DARP to demodulate the GMSK signal. With BIC, thereceiver has no knowledge of the structure of any interfering signalsthat may be received at the same time that the desired signal isreceived. Since the receiver is effectively “blind” to anyadjacent-channel interferers, the method attempts to suppress theinterfering component as a whole. The GMSK signal is demodulated fromthe wanted carrier by the BIC method. BIC is most effective when usedfor GMSK-modulated speech and data services and can be used inasynchronous networks.

A DARP capable remote station equalizer/detector 426 of the embodimentsdescribed herein and in the accompanying drawings also performs CCIcancellation prior to equalization, detection, etc. Theequalizer/detector 426 in FIG. 2 provides demodulated data. CCIcancellation normally is available on a base station 110, 111, 114.Also, remote stations 123-127 may or may not be DARP capable. Thenetwork may determine whether a remote station is DARP capable at theresource assignment stage, a starting point of a call for a GSM remotestation (e.g. mobile station) 123-127.

FIG. 7 of the accompanying drawings shows example arrangements for datastorage within a memory subsystem which might reside within a basestation controller (BSC) of a cellular communication system 100. Table1001 of the figure is a table of values of frequency channel numbers(FCN) assigned to remote stations 123-127, the remote stations 123-127being numbered. Table 1002 of the figure is a table of values of timeslots wherein remote station numbers 123-127 are shown against time slotnumber. It can be seen that time slot number 3 is assigned to remotestations 123, 124 and 229. Similarly table 1003 shows a table of dataallocating training sequences (TSCs) to remote stations 123-127.

Table 1005 of the figure shows an enlarged table of data which ismulti-dimensional to include all of the parameters shown in tables 1001,1002, and 1003 just described. It will be appreciated that the portionof table 1005 shown in the figure is only a small part of the completetable that would be used. Table 1005 shows in addition to the allocationof frequency allocation sets, each frequency allocation setcorresponding to a set of frequencies used in a particular sector of acell or in a cell. In Table 1005, frequency allocation set f1 isassigned to all remote stations 123-127 shown in the table 1005 of thefigure. It will be appreciated that other portions of Table 1005, whichare not shown, will show frequency allocation sets f2, f3 etc. assignedto other remote stations 123-127. The fourth row of data shows no valuesbut repeated dots indicating that there are many possible values notshown between rows 3 and 5 of the data in table 1001.

FIG. 8 of the accompanying drawings shows a method for assigning achannel already in use by one remote station 123-127 to another remotestation 123-127.

Following the start of the method 1501, a decision is made as to whetherto set up a new connection between the base station 110, 111, 114 and aremote station 123-127 (block 1502). If the answer is NO, then themethod moves back to the start block 1501 and the steps above arerepeated. When the answer is YES (block 1502) then a determination ismade as to whether there is an unused channel, i.e. an unused time slotfor any either used or unused channel frequency (block 1503). If thereis an unused time slot then a new time slot is allocated (block 1504).The method then moves back to the start block 1501 and the steps aboveare repeated.

Eventually there is no longer an unused time slot (because all timeslots are already used or allocated for connections), and therefore theanswer to the question of block 1503 is NO, and the method moves toblock 1505. In block 1505 a used time slot is selected for the newconnection to share with an existing connection.

A first used time slot (channel) having been selected for the newconnection to share along with an existing connection. The existingconnection uses a first training sequence. A second training sequence,different from the first training sequence, is then selected for the newconnection in block 1506. The method then moves back to the start block1501 and the steps above are repeated.

FIG. 9 of the accompanying drawings is a schematic diagram of apparatuswherein the method represented by FIG. 8 resides in a base stationcontroller 600. Within the base station controller 600 are controllerprocessor 660 and memory subsystem 650. The steps of the method may bestored in software 680, in memory 685, in memory subsystem 650 or withinsoftware in memory residing in controller processor 660, or withinsoftware or memory in the base station controller 600, or within someother digital signal processor (DSP) or in other forms of hardware. Thebase station controller 600 is connected to the mobile switching centre610 and also to base stations 620, 630 and 640.

Shown within memory subsystem 650 are parts of three tables of data 651,652, 653. Each table of data stores values of a parameter for a set ofremote stations 123, 124 indicated by the column labeled MS. Table 651stores values of training sequence code. Table 652 stores values fortime slot number TS. Table 653 stores values of channel frequency CHF.It can be appreciated that the tables of data could alternatively bearranged as a multi-dimensional single table or several tables ofdifferent dimensions to those shown in the figure.

The controller processor 660 communicates via data bus 670 with memorysubsystem 650 in order to send and receive values for parameters to/frommemory subsystem 650. Within controller processor 660 are containedfunctions that include a function 661 to generate an access grantcommand, a function 662 to send an access grant command to a basestation 620, 630, 640, a function 663 to generate a traffic assignmentmessage, and a function 664 to send a traffic assignment message to abase station 620, 630 or 640. These functions may be executed usingsoftware 680 stored in memory 685.

Within the controller processor 660, or elsewhere in the base stationcontroller 600, there may also be a power control function 665 tocontrol the power level of a signal transmitted by a base station 620,630 or 640.

It can be appreciated that the functions shown as being within basestation controller 600, namely memory subsystem 650 and controllerprocessor 660 could also reside in the mobile switching centre 610. Someor all of the functions described as being part of base stationcontroller 600 could equally well reside in one or more of base stations620, 630 or 640.

Phase Shift

The absolute phase of the modulation for the two signals transmitted bythe base station 110, 111, 114 may not be identical. In order to servean additional user using the same channel (co-TCH), in addition toproviding more than one TSC the network may phase shift the data symbolsof the signal for the new co-channel (co-TCH) remote station withrespect to the signal for the already-connected co-channel remotestation(s). If possible the network may provide evenly spaced phaseshift, thus improving receiver performance. For one example of two userssharing a channel, the phase difference for one user relative to anotheruser could be 90 degrees apart. For another example in which three usersshare a channel, the phase difference for one user relative to anotheruser could be 60 degrees apart. The phase shift for four users could be45 degree apart. As stated above, the users will each use a differentTSC.

Thus, for improved DARP performance, the two signals intended for thetwo different remote stations 123, 124 may ideally be phase shifted byπ/2 for the best channel impulse response, but a phase shift less thanthis will also provide adequate performance.

To provide the two signals so that their phases are offset from eachother by 90 degrees, the first transmitter 1120 modulates the twosignals at 90 degrees phase shift to each other, thus further reducinginterference between the signals due to phase diversity.

In this way, the transmitting apparatus 1200 provides means at the basestation 620, 920 for introducing a phase difference betweencontemporaneous signals using the same time slot on the same frequencyand intended for different remote stations 123, 124. Such means can beprovided in other ways. For example, separate signals can be generatedin the transmitting apparatus 1200 and resulting analogue signals can becombined in a transmitter front end by passing one of them through aphase shift element and then simply summing the phase shifted andnon-phase shifted signals.

Power Control Aspects

Table 2 below shows example values of channel frequency, time slot,training sequence and received signal power level for signalstransmitted by the two base stations 110 and 114, and received by remotestations 123 to 127, shown in FIG. 4.

TABLE 2

The rows 3 and 4 of the table, outlined by a bold rectangle, show bothremote station 123 and remote station 124 using channel frequency havingindex 32 and time slot 3, for receiving a signal from base station 114but the remote stations 123, 124 are allocated different trainingsequences TSC2 and TSC3 respectively. Similarly, rows 9 and 10 also showthe same channel frequency and time slot being used for two remotestations 125, 127 to receive signals from the same base station 110. Itcan be seen that in each case the received power levels of the wantedsignals are substantially different for the two remote stations 125, 127(−101 and −57 dBm respectively).

The highlighted rows 3 and 4 of Table 3 show that base station 114transmits a signal for remote station 123 and also transmits a signalfor remote station 124. The received power levels of the wanted signalsare substantially different for the two remote stations 123, 124. Thereceived power level at remote station 123 is −67 dBm whereas thereceived power level at remote station 124 is −102 dBm. Rows 9 and 10 ofTable 3 show that base station 110 transmits a signal for remote station125 and also transmits a signal for remote station 127. The receivedpower level at remote station 125 is −101 dBm whereas the received powerlevel at remote station 127 is −57 dBm. The large difference in powerlevel, in each case, could be due to different distances of the remotestations 125, 127 from the base station 110. Alternatively, thedifference in power levels could be due to different path losses ordifferent amounts of multi-path cancellation of the signals, between thebase station 10, 111, 114 transmitting the signals and the remotestation 123-127 receiving the signals, for one remote station 123-127 ascompared to the other remote station 123-127.

Although this difference in received power level for one remote station123-127 compared to the other remote station 123-127 is not intentionaland not ideal for cell planning, it does not compromise the operation ofthe embodiments described herein and in the accompanying drawings.

A remote station 123-127 having DARP capability may successfullydemodulate either one of two co-channel, contemporaneously receivedsignals, so long as the amplitudes or power levels of the two signalsare similar at the remote station's 123-127 antenna. This is achievableif the signals are both transmitted by the same base station 110, 111,114 and the transmitted power levels of the two signals aresubstantially the same. Each of a first and second remote stations123-127 receives the two signals at substantially the same power level(say within 6 dB of each other) because the path losses for the twosignals between the base station and the first remote station aresimilar, and the path losses for the two signals between the basestation and the second remote station are similar. The transmittedpowers are similar if either the base station 110, 111, 114 is arrangedto transmit the two signals at similar power levels, or the base station110, 111, 114 transmits both signals at a fixed power level. Thissituation can be illustrated by further reference to Table 2 and byreference Table 3.

While Table 2 shows remote stations 123, 124 receiving from base station114 signals having substantially different power levels, on closerinspection it can be seen that, as shown by rows 3 and 5 of Table 2,remote station 123 receives two signals from base station 114 at thesame power level (−67 dBm), one signal being a wanted signal intendedfor remote station 123 and the other signal being an unwanted signalwhich is intended for remote station 124. The criteria for a remotestation 123-127 to receive signals having similar power levels is thusshown as being met in this example. If mobile station 123 has a DARPreceiver, it can, in this example, therefore demodulate the wantedsignal and reject the unwanted signal.

Similarly, it can be seen by inspecting rows 4 and 6 of Table 2 (above)that remote station 124 receives two signals sharing the same channeland having the same power level (−102 dBm). Both signals are from basestation 114. One of the two signals is the wanted signal, for remotestation 124 and the other signal is the unwanted signal which isintended for use by remote station 123.

To further illustrate the above concepts, Table 3 is an altered versionof Table 2 wherein the rows of Table 2 are simply re-ordered. It can beseen that remote stations 123 and 124 each receive from one base station114 two signals, a wanted and an unwanted signal, having the samechannel and similar power levels. Also, remote station 125 receives fromtwo different base stations 110, 114 two signals, a wanted and anunwanted signal, having the same channel and similar power levels.

TABLE 3

It is possible for a base station 110, 111, 114 to maintain a call withtwo remote stations 123-127 using the same channel, such that a firstremote station 123-127 has a DARP-enabled receiver and a second remotestation 123-127 does not have a DARP-enabled receiver. The amplitudes ofsignals received by the two remote stations 124-127 are arranged to bedifferent by an amount which is within a range of values (in one exampleit may be between 8 dB and 10 dB) and also arranged such that theamplitude of the signal intended for the DARP-enabled remote station islower than the amplitude of the signal intended for the non-DARP-enabledremote station 124-127.

An advantage with MUROS enabled networks is that the base station 110,111, 114 may use two or more training sequences per timeslot instead ofonly one so that both signals may be treated as desired signals. Thebase station 110, 111, 114 transmits the signals at suitable amplitudesso that each remote station receives its own signal at a high enoughamplitude and the two signals maintain an amplitude ratio such that thetwo signals corresponding to the two training sequences may be detected.This feature may be implemented using software stored in memory in thebase station 110, 111, 114 or BSC 600. For example, remote stations123-127 are selected for pairing based on their path losses beingsimilar and based on existing traffic channel availability. However,MUROS can still work if the path losses are very different for oneremote station than for the other remote station 123-127. This may occurwhen one remote station 123-127 is much further away from the basestation 110, 111, 114 than for the other remote station.

Regarding power control there are different possible combinations ofpairings. Both remote stations 123-127 can be DARP capable, or only oneDARP capable. In both cases, the received amplitudes or power levels atthe mobiles 123-127 may be within 10 dB of each other. However if onlyone remote station 123-127 is DARP capable, a further constraint is thatthe non-DARP remote station 123-127 has its wanted (or desired) firstsignal higher than the second signal (in one example, at least 8 dBhigher than the second signal). The DARP capable remote station 123-127receives its second signal no more than a lower threshold below thefirst signal (in one example, it is no lower than 10 dB below the firstsignal). Hence in one example, the amplitude ratio can be 0 dB to ±10 dBfor DARP/DARP capable remote stations 123-127 or an 8 dB to 10 dB highersignal for a non-DARP/DARP pairing in favor of the non-DARP remotestation 123-127. Also, it is preferable for the base station 110, 111,114 to transmit the two signals so that each remote station 123-127receives its wanted signal at a power level above its sensitivity limit.(In one example, it is at least 6 dB above its sensitivity limit). So ifone remote station 123-127 has more path loss, the base station 110,111, 114 transmits that remote station's 123-127 signal at power levelor amplitude appropriate to achieve this. This sets the transmittedpower level. The required difference between the levels of the twosignals then determines the absolute power level of that other signal.

FIG. 10 of the accompanying drawings shows receiver architecture for aremote station 123-127 having enhanced co-channel rejection capability.The receiver is adapted to use either the single antenna interferencecancellation (SAIC) equalizer 1105, or the maximum likelihood sequenceestimator (MLSE) equalizer 1106. The SAIC equalizer is preferred for usewhen two signals having similar amplitudes are received. The MLSEequalizer is typically used when the amplitudes of the received signalsare not similar, for example when the wanted signal has an amplitudemuch greater than that of an unwanted co-channel signal.

Selecting a Receiving Apparatus for Co-Channel Operation

As described above, MUROS allows more than one user on the same trafficchannel (TCH) which results in enhanced capacity. This can be achievedby taking advantage of the DARP capability of remote stations 123-127. ADARP remote station 123-127 offers more pairing opportunities whenpaired with another DARP remote station 123-127 because the DARP remotestation can tolerate an unwanted co-channel signal at a higher powerlevel than that of its own wanted signal, as explained above. However itis still possible to pair a non-DARP remote station 123-127 with a DARPremote station 123-127 for co-channel (i.e. MUROS) operation, as alsodescribed above. Therefore, it is advantageous to be able to select aremote station 123-127 for MUROS operation when it is not known whetheror not the remote station 123-127 has DARP capability. It is alsoadvantageous to be able to select a remote station 123-127 for MUROSoperation without the need for a message to be transmitted indicatingthat the remote station has MUROS capability. This is because the systemcannot produce such a message if the remote station 123-127 is aso-called legacy remote station which does not indicate that it has DARPcapability. Apparatus and methods for selecting either a DARP or anon-DARP remote station 123-127 are described below.

If a transmitter is to transmit two co-channel signals, one for each oftwo receivers, then knowledge about each receiver's co-channel rejectioncapability is used, in order firstly to decide if both receivers arecapable of handling the two co-channel signals and secondly to set thepower levels of the transmitted signals in the correct ratio to ensureeach receiver can handle the two signals. For example, one receiver maybe non-DARP or one receiver may be further away from the transmitterthan the other receiver, and both these factors determine the mostsuitable power levels of the transmitted signals, as described above.

A base station 110, 111, 114 may identify a remote station's 123-127DARP capability by requesting the remote station's 123-127 classmark. Aclassmark is a declaration from a remote station 123-127 to a basestation 110, 111, 114 of its capabilities. This is described, forexample, in 24.008 of TS10.5.1.5-7 in the GERAN standards. Currently,the standards define a classmark indicative of a remote station's123-127 DARP capability but so far, no MUROS classmark or classmarkindicating support of new training sequences has been defined.

Additionally, despite the definition of a DARP classmark in thestandards, the standards do not require the remote station 123-127 tosend the classmark to the base station 110, 111, 114. In fact, manymanufacturers do not design their DARP-capable remote stations 123-127to send the DARP classmark to the base station 110, 111, 114 on callsetup procedures for fear that their remote stations 123-127 willautomatically be assigned to noisier channels by the base station 110,111, 114, thereby potentially degrading the communication from thatremote station 123-127. It is desirable to identify whether or not alegacy remote station 123-127 is MUROS capable without using theclassmark. It is currently not possible to identify with any certainty,whether a remote station 123-127 is MUROS-capable or even DARP-capable,without a prior knowledge of a remote station's DARP capability beingsignaled.

A base station 110, 111, 114 may identify MUROS-capability in a remotestation 123-127 based on the International Mobile Equipment Identity(IMEI) of the remote station 123-127. The base station 110, 111, 114 mayestablish the remote station's 123-127 IMEI by requesting it directlyfrom the remote station 123-127. The IMEI is unique to the remotestation 123-127 and can be used to reference a database located anywherein the network, thereby identifying the model of mobile phone to whichthe remote station 123-127 belongs, and additionally its capabilitiessuch as DARP and MUROS. If the phone is DARP or MUROS capable, it willbe considered by the base station 110, 111, 114 as a candidate forsharing a channel with another suitable remote station 123-127. Inoperation, the base station 110, 111, 114 will build up a list of remotestations 123-127 currently connected to that base station 110, 111, 114which are DARP or MUROS capable.

However, DARP or MUROS capability alone may not be a sufficientcriterion for determining whether a particular remote station 123-127can share a TDMA slot on the same frequency with another remote station123-127.

One way of determining the interference rejection capability of a remotestation 123-127 is to send a discovery burst. This is a short radioburst in which a signal for the remote station 123-127 has a knowninterference pattern superimposed on it. The discovery burst comprises asignal containing a first traffic data for the remote station (e.g.basic speech) comprising a first predefined data sequence (e.g. a firsttraining sequence) and a second (co-channel) signal comprising seconddata comprising a second predefined data sequence (e.g. a secondtraining sequence), both signals at predefined power levels.

FIG. 11 of the accompanying drawings is a schematic diagram of (a) atransmitting apparatus 1200 and (b) a receiving apparatus 1240 suitablefor selecting a receiving apparatus for co-channel operation. Thetransmitting apparatus 1200 is configured to transmit two sets of dataat predetermined power levels on a single channel. The receivingapparatus 1240 is configured: to receive the transmitted data; tomeasure a characteristic of the received data; and to transmit a signalindicating the characteristic. The transmitting apparatus 1200 andreceiving apparatus 1240 are together suitable for selecting thereceiving apparatus 1240 for co-channel operation. The features of thetransmitting apparatus 1200 and receiving apparatus will now bedescribed in more detail.

The transmitting apparatus 1200 comprises: a first transmitter 1220; aselector comprising a processor 1215 and a memory 1216; a first receiver1217 coupled to the selector 1230, the first receiver configured toreceive a first signal indicating a measured characteristic oftransmitted data; and a third receiver 1218, coupled to the selector1230, configured to receive a second signal indicating a co-channelrejection capability of a receiving apparatus.

A first data source 1201 is configured to output first data. A firstmultiplexer 1203, coupled to the first data source 1201, receives thefirst data and is configured: to time division multiplex the first databy allocating a first time slot to the first data; and to output themultiplexed first data.

A first power adjuster 1205, coupled to the first multiplexer 1203, isconfigured to adjust the power level of the multiplexed first data toproduce first power-adjusted data. A first modulator 1207, coupled tothe first power adjuster 1205, is configured to modulate the firstpower-adjusted data onto a first channel frequency to produce firstmodulated data 1209. A first amplifier 1211, coupled to the firstmodulator 1207, is configured to transmit the first modulated data 1209to produce transmitted first data 1213.

A second data source 1202 is configured to output second data. A secondmultiplexer 1204, coupled to the second data source 1202, receives thesecond data and is configured: to time division multiplex the seconddata by allocating a second time slot to the second data; and to outputthe multiplexed second data.

A second power adjuster 1206, coupled to the second multiplexer 1204, isconfigured to adjust the power level of the multiplexed second data toproduce second power-adjusted data. A second modulator 1208, coupled tothe second power adjuster 1206, is configured to modulate the secondpower-adjusted data onto a second channel frequency to produce secondmodulated data 1210. A second amplifier 1212, coupled to the secondmodulator 1208, is configured to transmit the second modulated data 1210to produce transmitted second data 1214. A combiner 1219, coupled to thefirst and second amplifiers 1211, 1212, is operable to combine thetransmitted first and second data 1213, 1214, to produce combinedtransmitted first and second data. Optionally, the transmitted first andsecond data 1213, 1214 are each transmitted without being combined.

The receiving apparatus 1240 comprises a second receiver 1241 operableto receive the transmitted first and/or second data and to outputreceived data. A demodulator 1242, coupled to the second receiver 1241,is operable to demodulate the received data to produce demodulated data.A demultiplexer 1243, coupled to the demodulator 1242, is operable totime division demultiplex the demodulated data to produce demultiplexeddata. A data quality estimator 1244, coupled to the demultiplexer 1243,is operable to measure a characteristic of the data and to output anindication of the measured characteristic. For example, the data qualityestimator 1244 may measure the bit error rate (BER), or the bit errorprobability (BEP) of the data. A second transmitter 1245, coupled to thequality estimator 1244, is operable to transmit a first signalcomprising the indication of the measured characteristic.

The receiving apparatus also 1240 comprises a second processor 1247,configured to communicate with and control operation of: thedemultiplexer 1243, data quality estimator 1244, and second transmitter1245. The second processor 1247 may be configured to control theoperation of the second receiver 1241, and the demodulator 1242. Asecond memory 1248, coupled to the second processor 1247, is configuredto store, and transfer to the second processor 1247, data includinginstructions for the processor 1247 to use in controlling the operationof elements as described above.

The receiving apparatus 1240 also comprises a third transmitter 1246,coupled to the second processor 1247, operable to transmit a secondsignal comprising an indication of a co-channel rejection capability ofthe receiving apparatus 1240.

The transmitting apparatus 1200 further comprises a first receiver 1217and a third receiver 1218, each coupled to the selector 1230. The firstreceiver 1217 is operable to receive the first signal transmitted by thesecond transmitter 1245 of the receiving apparatus 1240 and to outputthe indication of the measured characteristic to the selector 1230. Thethird receiver 1218 is operable: to receive the second signaltransmitted by the third transmitter 1246 of the receiving apparatus1240; and to output the indication of the co-channel rejectioncapability to the selector 1230.

The selector 1230 is arranged to select the receiving apparatus 1240 forco-channel operation depending on the measured characteristic, and/or toselect the receiving apparatus 1240 for co-channel operation dependingon the co-channel rejection capability of the receiving apparatus 1240.

The Bit Error Probability (BEP) is measured at the remote station123-127. (Other parameters indicating ability of the remote station123-127 to reject interference may also be used as discussed below). TheBEP value is transmitted in the remote station's 123-127 periodic reportback to the base station 110, 111, 114. In the GERAN standards, forexample, the BEP is represented by the values 0-31 with 0 correspondingto a probability of bit error of 25% and 31 corresponding to aprobability of 0.025%. In other words, the higher the BEP, the greaterthe ability of the remote station 123-127 to reject interference. TheBEP is reported as part of an “enhanced measurement report” or “extendedreport.” R99 and later phones may have the capability to report BEP.

Once the burst has been sent, if the BEP of the remote station 123-127falls below a given threshold, the remote station 123-127 may beconsidered to be unsuitable for MUROS operations. In simulations, a BEPof at least 25 has been shown to be an advantageous choice of threshold.It is noted that the BEP is derived by sending a burst over the channeland measuring the number of errors occurring in the burst at the remotestation 123-127.

However, the BEP on its own may not be an accurate enough measure of thequalities of the remote station 123-127 and the channel, particularly ifthere is a dramatic variation of error frequency across the burst. Itmay therefore be preferable to base the MUROS operation decision on themean BEP taking account of the co-variance of the BEP (CVBEP). These twoquantities are mandated by the standards as being present in the reportwhich the remote station 123-127 sends to the base station 110, 111,114.

Alternatively, the determination of whether the remote station issuitable for co-channel operation could be based on the RxQual parameterreturned to the base station 110, 111, 114 by the remote station 123-127for one SACCH period (0.48 ms). RxQual is a value between 0 and 7 whereeach value corresponds to an estimated number of bit errors in a numberof bursts i.e. the bit error rate (BER, see 3GPP TS 05.08). The higherthe bit error rate, the higher is RxQual. Simulations have shown anRxQual of 2 or lower to be an advantageous choice of threshold for MUROSoperation.

Alternatively, the parameter RxLev may be used as a selection criterion.RxLev indicates the average signal strength received in dBm. This wouldalso be reported by the remote station 123-127 after the discoveryburst. An RxLev of at least −100 dBm has been shown to be advantageous.While particular criteria for MUROS pairing have been described, itwould be plain to the skilled person that many other criteria could beused instead or in combination with those identified above.

FIG. 12A of the accompanying drawings is a schematic diagram showingsequences of data frames each containing, or not containing, discoverybursts comprising co-channel data. Three sets of 29 consecutive dataframes contain discovery bursts in some of the frames. Time isrepresented as the horizontal axis on the drawing. Each frame istransmitted during a frame period. Each such frame period is separatedfrom an adjacent frame period by a small vertical line on the drawing.Each frame has a frame index, from 0 to 25, as shown.

A first set of frames 1401 comprises 29 consecutive frames. During afirst time interval 1410, corresponding to a frame period of a firstframe having index zero (the frame shown as a shaded box labeled zero onthe drawing), a discovery burst is transmitted by the transmittingapparatus 1200 on a first channel. The first channel comprises time slot3 of the first frame. Normal traffic bursts are transmitted during allthe remaining seven of the eight time slots of the first frame, i.e. ondifferent channels to the first channel. The transmitting apparatus maytransmit the discovery burst based on a signal which the transmittingapparatus has received, the signal indicating a measured characteristicof received data.

For example, a receiving apparatus, which has received data transmittedon the first channel by the transmitting apparatus, may send a signalindicating that the measured characteristic of the received data (e.g.the BEP) has a prescribed value. The measured characteristic may have aprescribed value i.e. it may be within a prescribed range of values orit may be above some value. If the measured characteristic has theprescribed value, then the discovery burst is transmitted. The receiveddata may be either data which has been transmitted in a normal burst, ordata which has been transmitted in a discovery burst.

During a second time interval 1411, corresponding to the next twentyfive consecutive frames having indices of 1 to 25 inclusive, normaltraffic bursts are transmitted in all eight time slots of each frame,each such frame having no discovery burst. Starting with the nextconsecutive frame, indexed zero, the process described above for frames0 to 25 is repeated.

Each time a frame is transmitted a receiving apparatus 1240 receives theframe of data and then measures a characteristic of the data (e.g. BEP).The receiving apparatus 1240 transmits a first signal 1260 indicatingthe measured characteristic.

The transmitting apparatus 1200 selects, or does not select, thereceiving apparatus 1240 for co-channel operation depending on themeasured characteristic.

The transmitting apparatus 1200 may select or not select the receivingapparatus 1240 depending on the measured characteristic of a singleframe (e.g. frame indexed zero), or depending on the measuredcharacteristic of several frames. The frame(s) for which thecharacteristic is measured could include, or not include, a framecontaining a discovery burst.

If the transmitting apparatus 1200 does not select the receivingapparatus, then the transmitting apparatus 1200 may then transmit, for aprescribed period, only normal traffic bursts and not discovery bursts.

If, on the other hand, the transmitting apparatus 1200 selects thereceiving apparatus 1240, then the transmitting apparatus 1200 may againtransmit, for a prescribed period, one or more discovery bursts. Thetransmitting apparatus 1200 may transmit a greater portion of framescontaining discovery bursts than just described, as set out below.

In a second set of frames 1402, the process described above for thefirst set of frames is carried out, except that a discovery burst istransmitted in both the frame indexed 0 and also the frame indexed 1.Thus the transmitting apparatus 1200 transmits a greater proportion offrames containing discovery bursts, compared to the case discussed abovefor the set of frames 1401.

In a third set of frames 1403, the process described above for the firstset of frames 1401 is carried out, except that a discovery burst istransmitted in the frames indexed 0, 1 and 2. Thus the transmittingapparatus 1200 transmits a greater proportion of frames containingdiscovery bursts, compared to the cases discussed above for the sets offrames 1401 or 1402.

The transmitting apparatus 1200 may continue to increase the proportionof frames containing discovery bursts frames it transmits, in relationto the total number of frames transmitted, until either all framescontain discovery bursts (hence co-channel data), or the receivingapparatus 1240 transmits a signal indicating that the measuredcharacteristic falls outside a predefined range. For example, the BEPmay be less than a predefined value.

Multiple frames containing discovery bursts may be transmittedconsecutively in groups, as described above. Alternatively, the multipleframes may be transmitted non-consecutively. For example, a discoveryburst may be transmitted in frames indexed 0 and 4, or several discoverybursts may be interspersed between sets of normal bursts.

FIG. 12B of the accompanying drawings is a further schematic diagramshowing sequences of data frames each containing, or not containing,discovery bursts comprising co-channel data. Such sequences would besuitable for use in a GERAN system.

Each sequence of frames, 1404 to 1408, is a sequence of frames of SACCHdata transmitted by the transmitting apparatus in a SACCH period. Thesequence of frames 1404 is transmitted in SACCH 1 period (labeled SACCH1), the sequence of frames 1405 is transmitted in SACCH 2 period(labeled SACCH 2) and so on.

Referring to each SACCH period, the first frame furthest to the left onthe figure is labeled S, and is a SACCH signaling frame. The next framehas frame index 48 and contains a discovery burst. The frame with index48 thus comprises a first time interval during which a discovery burstis transmitted. The first time interval may be considered as the periodof the frame containing the discovery burst, or it may be considered asthe time of duration of the discovery burst itself, i.e. a time slot.For the sake of simplicity, the first time interval is consideredhereinafter as the period of the frame containing the discovery burst.

Frame 49 of the SACCH 1 period and the remainder of frames in SACCH 1period contain no discovery burst.

During SACCH 2 period 1405, the transmitting apparatus 1200 transmitsSACCH data which does not comprise any discovery burst. The receivingapparatus receives the transmitted SACCH data. During a periodcorresponding to SACCH 2 period, the receiving apparatus 1240 transmitsa first signal 1260. The first signal comprises a measuredcharacteristic (e.g. BEP) of data which has been transmitted bytransmitting apparatus during SACCH 1 period and received by thereceiving apparatus 1240. The first signal comprises a message in aframe corresponding to a frame labeled S (e.g. the frame preceding frame48 or the frame preceding frame 71).

The transmitting apparatus continues to transmit frames containingnormal bursts (not discovery bursts) until, in frame indexed 48 of SACCH3 period, the transmitting apparatus transmits a frame of datacontaining a discovery burst. Therefore the interval of time betweenframe 48 of SACCH 1 period and frame 48 of SACCH 3 period is the secondtime interval discussed above, during which no discovery bursts aretransmitted. The second time interval may be defined as the timeinterval between the end of the discovery burst in frame 48 of SACCHperiod 1 and the beginning of the discovery burst in frame 48 of SACCHperiod 3. Alternatively the second time interval may be defined as thetime interval between the end of frame 48 of SACCH period 1 and thebeginning of frame 48 of SACCH period 3. A discovery burst istransmitted in both these frames.

During SACCH 3 period 1406, the transmitting apparatus: transmits aframe indexed 48 which contains a discovery burst; then transmits threeframes indexed 49, 50 and 51 which contain no discovery burst; and thentransmits a frame indexed 52 which contains a discovery burst. Thetransmitting apparatus then transmits frames containing normal burstsuntil, in frame indexed 48 of SACCH 5 period 1408, the transmittingapparatus transmits a frame of data containing a discovery burst.

The transmitting apparatus transmits one more frame containing adiscovery burst during SACCH 3 period than for SACCH 1 period, dependingon the measured characteristic which is transmitted by the receivingapparatus and received by the transmitting apparatus during a periodcorresponding to SACCH 2 period.

Similarly, the transmitting apparatus transmits, during SACCH 5 period,three frames which each contain a discovery burst i.e. it transmits onemore frame containing a discovery burst during SACCH 5 period than forSACCH 3 period, depending on the measured characteristic which istransmitted by the receiving apparatus and received by the transmittingapparatus during a period corresponding to SACCH 4 period.

This process of adding a further frame containing a discovery burst fora later SACCH period may continue until either the measuredcharacteristic of received data no longer meets predefined criteria oruntil a predetermined proportion of transmitted frames contain discoverybursts (e.g. all transmitted frames).

Table 4 below is a tabular listing of indexed SACCH data frames, fortwelve SACCH periods. SACCH 1 to SACCH 8 periods are consecutive andSACCH 21 to SACCH 24 periods are consecutive. SACCH 9 to SACCH 20periods are not shown, for simplicity. Frames containing a discoveryburst are shown as having bold text and borders.

TABLE 4

During SACCH 1 period, the transmitting apparatus transmits frames ofwhich frame 48 contains a discovery burst and the remaining frames donot contain a discovery burst.

During SACCH 2 period, the measured characteristic of the datatransmitted in the SACCH 1 period is transmitted by the receivingapparatus and received by the transmitting apparatus during a periodcorresponding to SACCH 4 period. The measured characteristic meets thepredefined criteria.

Because the measured characteristic meets the predefined criteria,during SACCH 3 period, the transmitting apparatus transmits frames ofwhich frame 48 and frame 52 contain a discovery burst and the remainingframes do not contain a discovery burst. The process of adding framescontaining discovery bursts continues, as shown for the subsequent SACCH4 to 13 periods.

Each time the transmitting apparatus receives the measuredcharacteristic, the transmitting apparatus selects, or does not select,the receiving apparatus for co-channel operation and, depending on themeasured characteristic, the transmitting apparatus may transmit agreater proportion of frames containing discovery bursts.

It can be seen from the figure that during SACCH 13 period, alternateframes contain discovery bursts.

A final selection of the receiving apparatus results in the transmittingapparatus transmitting co-channel data during a predetermined proportionof the transmitted frames, for example all of the frames or apredetermined maximum number of frames.

After a first receiving apparatus is selected for co-channel operation,a second receiving apparatus may be selected using the proceduredescribed above except that, to select the second receiving apparatus,discovery bursts are transmitted on the second channel, the secondchannel being for the data intended for the second receiving apparatus.Described above is the transmitting of discovery bursts on the firstchannel to select the first receiving apparatus.

Alternatively, both the first and second receiving apparatus may beselected substantially simultaneously, whereby each of the first andsecond data are transmitted on each channel.

Testing a Traffic Channel

Described below are methods and apparatus which illustrate how the abovefeatures may be applied to a pair of remote stations 123-127 operatingusing MUROS/VAMOS in a GSM or GERAN communications system.

The network may evaluate a plurality of traffic channel (TCH) candidateswhich two or more remote stations 123-127 may potentially use as a MUROSTCH. The selected TCH may be the TCH currently in use by a pair of users(for example when the users are served by different cells or sectors),or it may be an unused TCH that is known to have good metrics (seebelow). Subsequently, one of the remote stations 123-127 may be movedonto another TCH which is already in use. To increase the capacity of acell, the network may consider a number of current remote stations123-127 to potentially be operated in MUROS mode. Many pairs of remotestations 123-127 may be tested in parallel, possibly by the base stationradio management entity. The network may enable the extended report andrely on the remote stations 123-127 reporting their BEP if they are R99or later. If the remote stations 123-127 are pre-R99, the network mayrely on the remote stations 123-127 transmitting signals indicatingRxqual and RxLev values.

Before MUROS is fully utilized on a TCH (e.g. during every or mosttraffic data frames), the TCH may be tested as follows. A discoveryburst is transmitted by the base station 110, 111, 114 in place of anormal traffic (e.g. speech) burst. If the report returned by the remotestation 123-127 to the base station 110, 111, 114 (e.g., enhancedmeasurement report, or extended report) indicates that the remotestation 123-127 can sufficiently reject the interference caused by theco-channel signal, more discovery bursts can be sent. In one example,the discovery bursts may be sent at regular intervals, such as everySACCH period. This burst may be referred to as a MUROS discovery burst.The discovery bursts can vary in following aspects with regards to thenormal (non-discovery) traffic bursts.

The amplitudes of the discovery bursts, may vary. The discovery burstsmay consist of a few bits/symbols of a burst to half a burst or a wholeburst.

The amount of discovery bursts sent may range from one to a few, andfrom non-consecutive discovery bursts to consecutive bursts.

The modulation types of the discovery burst may be different to themodulation type of the normal traffic bursts.

The modulation types of the discovery burst may vary (i.e., QPSK,alpha-QPSK, linear sum of two GMSK and high order modulations, such as8PSK, 16QAM).

If discovery bursts are added gradually the performance of remotestations 123-127 is not degraded unacceptably during calls. It ispreferable to determine a remote station's 123-127 MUROS capabilitywithout disturbing the communication. A GERAN system can make thisdetermination because the system was designed to have some margin tocombat fading since the system may not have either a fast, or afine-step, feedback loop for physical layer power control. For aDARP-enabled remote station, such a margin is large enough that it ispossible to use traffic bursts for transmitting discovery bursts to theDARP remote station, for the purpose of setting up another call.

Tables 4 and 5 below show listings of consecutive transmitted frames ofdata transmitted by the transmitting apparatus on a first channel(channel 1) and a second channel (channel 2). The frames are indexedfrom 0 to 25, the sequence of frame indices then repeating from 0 to 6.

TABLE 4 Frame index Channel 1 Channel 2 0 D1&D2 D2 1 D1 D2 2 D1 D2 3 D1D2 4 D1 D2 5 D1 D2 6 D1 D2 7 D1 D2 8 D1&D2 D2 9 D1&D2 D2 10 D1 D2 11 D1D2 12 D1 D2 13 D1 D2 14 D1 D2 15 D1 D2 16 D1&D2 D2 17 D1&D2 D2 18 D1&D2D2 19 D1 D2 20 D1 D2 21 D1 D2 22 D1 D2 23 D1 D2 24 D1 D2 25 D1 D2 0D1&D2 D2 1 D1&D2 D2 2 D1&D2 D2 3 D1&D2 D2 4 D1&D2 D2 5 D1&D2 D2 6 D1&D2D2

TABLE 5 Frame index Channel 1 Channel 2 0 D1&D2 D1&D2 1 D1 D2 2 D1 D2 3D1 D2 4 D1 D2 5 D1 D2 6 D1 D2 7 D1 D2 8 D1&D2 D1&D2 9 D1&D2 D1&D2 10 D1D2 11 D1 D2 12 D1 D2 13 D1 D2 14 D1 D2 15 D1 D2 16 D1&D2 D1&D2 17 D1&D2D1&D2 18 D1&D2 D1&D2 19 D1 D2 20 D1 D2 21 D1 D2 22 D1 D2 23 D1 D2 24 D1D2 25 D1 D2 0 D1&D2 D1&D2 1 D1&D2 D1&D2 2 D1&D2 D1&D2 3 D1&D2 D1&D2 4D1&D2 D1&D2 5 D1&D2 D1&D2 6 D1&D2 D1&D2

Referring to the second column of the tables above, headed channel 1,during a first time interval corresponding to frame indexed zero, afirst data D1 comprising a first data sequence, and a second(co-channel) data D2 comprising a second data sequence, are transmittedon a first channel (channel 1). During the first time interval, thesecond data is also transmitted on a second channel (channel 2).

The transmitted frames of data are received by the receiving apparatus1240. The receiving apparatus 1240 measures a characteristic of receiveddata, based on some or all received frame(s), and transmits a signalindicating the characteristic. The signal is received by thetransmitting apparatus 1200.

During a second time interval corresponding to frames indexed 1 to 7,the first data D1 (but not the second data D2) is transmitted on thefirst channel (channel 1) and the second data is transmitted on thesecond channel (channel 2). Optionally the second data is onlytransmitted on channel 2 during the first time interval. This wouldresult in loss of a portion of the second data on the second channel butit may be a simpler implementation. The transmitted frames may containno co-channel data either depending, or not depending on thecharacteristic.

Depending on the characteristic (e.g. if the measured BEP isacceptable), during a third time interval corresponding to framesindexed 8 and 9, the first data D1 and the second (co-channel) data D2are transmitted by the transmitting apparatus 1200 on the first channel(channel 1), and the second data is transmitted on a second channel(channel 2). Optionally the second data is only transmitted on channel 2during the first time interval.

During a fourth time interval corresponding to frames indexed 10 to 15,the first data D1 (but not the second data D2) is transmitted on thefirst channel (channel 1) and the second data is transmitted on thesecond channel (channel 2).

During a fifth time interval corresponding to frames indexed 16 to 18,the first data D1 and the second (co-channel) data D2 are transmitted onthe first channel (channel 1), and the second data is transmitted on asecond channel (channel 2).

During a sixth time interval corresponding to frames indexed 19 to 25,the first data D1 (but not the second data D2) is transmitted on thefirst channel (channel 1) and the second data is transmitted on thesecond channel (channel 2).

During a seventh time interval corresponding to frames indexed 0 to 6,the first data D1 and the second (co-channel) data D2 are transmitted onthe first channel (channel 1), and the second data is transmitted on asecond channel (channel 2).

Thus, depending on the measured characteristic of received data, thesecond data is either sent, or not sent, on the same channel as thefirst data. Additionally, as shown in table 4, the second data is senton the same channel as the first data during a time interval whichdepends on the measured characteristic of received data. For example, ifthe BEP reported for received frames 0 to 7 of table 4 (or for onlyframe 0) is within a predetermined range, then both first and second(co-channel) data are transmitted in frames 8 and 9. The time intervalfor sending co-channel data (i.e. the number of frames in this example)may be set to increase with time so long as the measured characteristicremains within the predetermined range and until a target proportion offrames contain co-channel data.

Thus, Table 4 shows a listing of consecutive transmitted frames of datain which: a portion of the frames transmitted on channel 1 containdiscovery bursts i.e. co-channel data (first data D1 for a firstreceiving apparatus and second data D2 for a second receivingapparatus); and all of the frames transmitted on channel 2 contain onlythe second data D2. The discovery bursts are used, as described above,to select, or not select, the first receiving apparatus.

Table 5 shows a listing of consecutive transmitted frames of data inwhich: a portion of the frames transmitted on channel 1 containdiscovery bursts and all of the frames transmitted on channel 2 containonly the second data D2; and additionally a portion of the framestransmitted on channel 2 contain discovery bursts. For simplicity, thediscovery bursts are shown as being transmitted in the same frames forboth channel 1 and channel 2, however the discovery bursts may betransmitted in different frames for channel 2 than for channel 1.

The discovery bursts as shown in table 5 are used as described above: toselect or not select the first receiving apparatus 1240; andadditionally to select or not select a second receiving apparatus 1240.

FIG. 13 of the accompanying drawings is a flow diagram of a method ofselecting a receiving apparatus 1240 for co-channel operation. A firstdata sequence is selected for first data (block 1601). The first datasequence comprises a first training sequence. A first power level isdetermined for transmitting the first data (block 1602). A second datasequence is selected for second data (block 1603). The second datasequence comprises a second training sequence. A second power level isdetermined for transmitting the second data (block 1604). The equalizer1105 of the receiving apparatus 1240 can use the first training sequenceto distinguish the first signal from the second signal, and can use thesecond training sequence to distinguish the second signal from the firstsignal.

The first and second data are transmitted on a first channel at therespective first and second power levels (block 1605). The transmitteddata is received in the receiving apparatus 1240 (block 1606) and acharacteristic of the data, BEP, is measured (block 1607). The receivingapparatus 1240 transmits a signal indicating the BEP (block 1608). Thetransmitting apparatus 1200 receives the signal (block 1609). Adetermination is made (block 1610) of whether the measuredcharacteristic meets predefined criteria, for example, does the BEP fallwithin a predefined limit? If the measured characteristic meets thepredefined criteria, the receiving apparatus 1240 is selected forco-channel operation (block 1611). If the measured characteristic doesnot meet the predefined criteria, the receiving apparatus 1240 is notselected for co-channel operation (block 1612) but is selected forsingle channel operation.

FIG. 14 of the accompanying drawings is a further flow diagram of amethod of selecting a receiving apparatus 1240 for co-channel operation.In this flow diagram, the steps are the same as those shown in FIG. 13,except that in block 1707, a characteristic of the first and second data(not only the first data) is measured. In block 1607 of FIG. 13, acharacteristic of only the first data is measured.

Selection of Speech Codec

Another consideration is that the CCI rejection of a DARP capable remotestation 123-127 will vary depending on which speech codec is used. Forexample, the ratio of transmitted powers for two paired remote stations123-127 may also be affected by the selection of codecs. For example, aremote station 123-127 using a low codec rate (such as AHS 4.75) wouldbe able to still operate while receiving less power (such as 2 dB) thanif the remote station 123-127 used a higher codec rate (such as AHS5.9),due to the coding gain. To find the better codecs for a pair of remotestations 123-127, a lookup table may be used to find suitable codecs forthe pair. Thus, the network may assign different downlink power levelsaccording to a) the distance from the base station 110, 111, 114 to theremote station 123-127, and b) the codecs used.

FIG. 15 of the accompanying drawings is a graph of FER performance underdifferent levels of signal-to-noise ratio (Eb/No) for different codecs.

FIG. 16 of the accompanying drawings is a graph of FER performance underdifferent levels of carrier to interference (C/I) for different codecs.

It may be better if the network finds co-channel users who are at asimilar distance from the base station 110, 111, 114. This is due to theperformance limitation of CCI rejection. If one signal is strongercompared to a weaker signal, the weaker signal may not be detected dueto the interference to the weaker signal by the stronger signal, if theratio of powers between weaker and stronger signal is too great.Therefore, the network may consider the distance from the base station110, 111, 114 to new users when assigning co-channels and co-timeslots.The following described procedures would allow the network to minimizethe interference to other cells.

Remote stations 123-127 may be selected as candidates for MUROSoperation based on, for example, the RxLev reported by each remotestation 123-127, and a traffic assignment (TA) made to the candidateMUROS remote stations 123-127. The network can dynamically determinepossible MUROS pairing groups of remote stations 123-127. For example,if a non-DARP capable remote station 123-127 is further away from aserving base station 110, 111, 114 than a DARP capable remote station123-127, it may be possible to pair the two remote stations 123-127 asdescribed above, such that the transmitted power levels are differentfor the two remote stations 123-127.

To dynamically pair groups of remote stations 123-127, the network maymaintain a dynamic database of the above information (e.g. range, RXLEV,etc.) for remote stations 123-127 in the cell and prepare to makechanges to the pairings when the RF environment changes. These changesinclude: new pairing, de-pairing and re-pairing either both of a pair ofremote stations 123-127, or just one of them. These changes aredetermined by: changes of power ratios between the paired MUROS remotestations 123-127; and also changes of codecs used by each MUROS caller.

As stated above, the metrics RXqual/BEP and RxLev may be used to measurethe effect of the discovery bursts. For those discovery bursts that havean associated increase of Rxqual or decrease of BEP (i.e. a degradedquality of received data at the remote station 123-127), the remotestation 123-127 at that moment may not be suitable for MUROS on the TCHcandidate on which the discovery bursts are transmitted. On the otherhand, if the BEP/Rxqual for the discovery burst is not much worse thanfor the normal bursts, then MUROS may be suitable for that candidateTCH.

For a 0 dB MUROS discovery burst (in which the co-channel data istransmitted at the same power level or amplitude as the normal trafficdata), the RxLev metric could have a 3 dB increase during the SACCHperiod when the discovery bursts are sent. Such a test may also be usedwith different codecs. For example, using codec ASH5.9 in a DARP capablephone 123-127, and assigning 0 dB MUROS power ratio between the twoMUROS signals in the discovery burst, would cause minimal degradation ofthe Rxqual/BEP metrics. On the other hand, a non-DARP capable phone123-127, in the same conditions, may indicate a drop in the Rxqualmetric even after only one discovery burst has been transmitted. Also,for a discovery burst which has a duration of one SACCH period (0.48sec), the RxLev metric may be 3 dB higher (due to 0 dB co-channel powerratio) than for the normal, non-discovery bursts.

For those remote stations 123-127 that are DARP capable, furtherinformation about their capability to pair with non-DARP capable andDARP capable phones 123-127 may be obtained. This information mayinclude: the power ratio between the co-TCH users; the codecs that canbe applied to each co-TCH users in their condition; or the trainingsequence to be used. Hence, a co-TCH can be adapted to wide range ofMUROS remote stations 123-127.

It is possible to obtain a sustainable power ratio between two remotestations 123-127 which can be paired on a MUROS co-TCH by a step by stepincrease in power of signal for the prospective co-TCH user and bygauging a suitable ratio where the metrics indicate an acceptableperformance. For those remote stations 123-127 where the power ratio isbelow a certain value, say −4 dB, it is possible to pair that remotestation 123-127 with a non-DARP capable phone 123-127. For those remotestations 123-127 where the power ratio is around 0 dB, then a DARPcapable remote station 123-127 can be used to pair with another DARPremote station.

For those remote stations 123-127 that are suitable or have been onMUROS calls, similar estimations apply so that the network may switchthe remote stations 123-127 back to normal operation when conditionsindicate to do so. The embodiments described herein and in theaccompanying drawings work with legacy remote stations 123-127, as thereis nothing new that a remote station 123-127 will do when paired with aMUROS capable remote station 123-127. The legacy DARP remote station123-127 just operates as if in normal operation without realizing that asmart network is using its DARP capability for good capacity gain in thecell.

Description of Prescribed Discovery Bursts

An ongoing voice call is kept alive and maintained by a SACCH. The basestation 110, 111, 114 relies on the remote station's 123-127 SACCHreport containing such information as, in one example, the value ofRXQual of a remote station 123-127, to decide what to do next. EachSACCH period/frame is 104 frames and 480 ms long. Enhanced power control(EPC) can reduce the period/frame length to 26 frames and 120 ms long.The remote station 123-127 is used to report previous SAACH periodperformance, so there is 480 ms or 120 ms delay. A call is dropped if anumber of SACCH reports are missing. An operator may set the value orthreshold of missing SAACH reports where a call is dropped. For example,losing 25 SACCH frames is likely to drop the call. On the other hand, acall won't be dropped if one SACCH frame is lost. A method may be usedto make a call drop decision.

Using EPC to determine if a remote terminal 123-127 is MUROS capable maybe quicker because its period/frame length is shorter. Both EPC and thenormal SACCH frame can be used by the network when sending discoverybursts to determine if a remote terminal 123-127 is MUROS capable. Beloware some examples of sending discovery bursts during a normal SACCHperiod to describe the points of operation. The same method may beapplied to an EPC case.

In order not to cause an unnecessary dropped call, the discovery burstsmay be applied lightly. i.e., one discovery burst per SACCH period, tostart with. Thus, at the beginning, only during 1 of the 104 frames in aSAACH period will a discovery burst be sent. The number of frames whendiscovery bursts are sent is then ramped up. MUROS may be applied tothose remote stations 123-127 that have no problem handling discoverybursts sent during all SACCH frames (104) in a SACCH period. In oneexample, it may be helpful to send discovery bursts to multiple SACCHframes to make sure the remote station 123-127 is good enough for MUROSoperation.

FIG. 17 is a flow diagram of a method of progressively increasing thenumber of discovery bursts within a SACCH period for a series of SACCHperiods. The method is low risk and avoids bad voice quality and droppedcalls.

Initially a base station 110, 111, 114 selects MUROS candidate remotestations from remote stations that report good Rxqual values, e.g.Rxqual=0 (step 1805 of FIG. 17).

The base station's transmitting apparatus sends just one discovery burstduring one frame of the 104 frame SAACH period (step 1810 of FIG. 17).For example one discovery burst is sent during TCH frame 48. The reasonsto start from frame 48 are: it is the first burst of a speech block; andthe base station 110, 111, 114 may need some time to process the lastSACCH data received from the remote station. Frame 48 is near the middleof the SAACH period. This gives the base station 110, 111, 114 enoughtime to analyze the remote station's 123-127 report during the lastSACCH period, before the next SAACH period starts.

During the next SACCH period, the base station 110, 111, 114 receives areport of the RxQual of the remote station 123-127 during the last SACCHperiod (step 1815). Other measured characteristics such as BEP or RxLevmay be identified in the report. No discovery bursts are sent in thenext SACCH period when a reference RxQual is reported to the basestation 110, 111, 114.

Next, the base station 110, 111, 114 determines if the RXQual isacceptable (step 1817). If the Rxqual is acceptable (for example, Rxqual<=1) the base station 110, 111, 114 transmits two discovery burstsduring the next SAACH period (step 1820). For example, discovery burstsmay be sent during TCH frames 48 and 52. This procedure avoids sendingtwo discovery bursts in one speech block (4 frames) at an early stage.If the discovery bursts cause speech data errors on this TCH, the speechquality is impacted less if the two discovery bursts are not sent in onespeech block.

The next SACCH period (SACCH (N+1) period) is used to report RxQual ofthe remote station 123-127 for this SACCH period (SACCH N period) to thebase station 110, 111, 114 (step 1825). If the RxQual is not acceptable,no more discovery bursts are sent (step 1822).

A progressively increasing number of discovery bursts are transmitted bythe base station 110, 111, 114 to the remote station 123-127 during aSAACH period until a threshold is reached. In one example, the thresholdis that the first burst of all 24 speech blocks in a SACCH framecomprises a discovery burst. In another example, discovery bursts aretransmitted during all 104 frames of a SAACH period. A possible sequenceof steps for transmitting discovery bursts is: 1:2:4:8:24, which is480×2×5=4800 msec. Therefore the first stage needs about 5 seconds todetermine the good MUROS candidates which will be put on a short list.

During the next SACCH period, the base station 110, 111, 114 receives areport of the RxQual of the remote station 123-127 during the last SACCHperiod (step 1825).

A determination is made of whether the RxQual is still acceptable (step1828) If the remote station's 123-127 Rxqual is still acceptable, then acheck is made of whether the threshold reached concerning the maximumnumber of discovery bursts to transmit during a SAACH (step 1830). IfRxQual is not acceptable, no more discovery bursts are transmitted (step1832). If the threshold is reached, the proportion of frames containingdiscovery bursts is no longer increased. (step 1835). If the thresholdis not reached, the number of discovery bursts in one SAACH period isincreased and the process returns to step 1825, to await the nextreporting of RXQual. (step 1840 of FIG. 17).

In one example, for those remote stations 123-127 that do not haveRxqual <3, discovery is stopped, and they are dropped from the shortlist of MUROS capable remote stations 123-127. The reference SACCHperiod may be a good reference period in which to compare a remotestation's 123-127 Rxqual with a remote station's 123-127 Rxqual during aSAACH period in which discovery bursts were sent. One reason is that theenvironment of the remote station 123-127 may change such that theRxQual deteriorates independently of any discovery bursts. That mayhappen when remote station 123-127 receives strong interference fromother remote stations 123-127 or the remote station's signal experiencesbad multipath fading.

The ¼ discovery burst rate (one discovery burst transmitted every 4^(th)frame) shown in SAACH period #11 is generally a good indication of MUROScandidates. From there, the base station 110, 111, 114 may transmittwice as many discovery bursts in SACCH period #13 (one discovery bursttransmitted every 2nd frame), or the base station 110, 111, 114 maychange the power level of the discovery bursts.

FIG. 18 of the accompanying drawings shows an apparatus for operating ina multiple access communication system to produce first and secondsignals sharing a single channel. A first data source 4001 and a seconddata source 4002 (for a first and a second remote station 123-127)produce first data 4024 and second data 4025 for transmission. Asequence generator 4003 generates a first sequence 4004 and a secondsequence 4005. A first combiner 4006 combines the first sequence 4004with the first 4024 data to produce first combined data 4008. A secondcombiner 4007 combines the second sequence 4005 with the second data4025 to produce second combined data 4009.

The first and second combined data 4008, 4009 are input to a transmittermodulator 4010 for modulating both the first and the second combineddata 4008, 4009 using a first carrier frequency 4011 and a first timeslot 4012. In this example, the carrier frequency may generated by anoscillator 4021. The transmitter modulator outputs a first modulatedsignal 4013 and a second modulated signal 4014 to a combiner 4022 whichcombines the modulated signals 4013, 4014 to provide a combined signalfor transmission. A RF front end 4015, connected to the combiner 4022,processes the combined signal by upconverting it from baseband to an RF(radio frequency) frequency. The combined upconverted signal is sent toantenna 4016 where the upconverted signal is transmitted viaelectromagnetic radiation. The combiner 4022 may be a part of either thetransmitter modulator 4010 or the RF front end 4015 or a separatedevice.

Having thus described the invention by reference to the embodiment shownin the accompanying drawings it is to be well understood that theembodiments in question are by way of example only and thatmodifications and variations such as will occur to those possessed withappropriate knowledge and skills may be made without departure from thespirit and scope of the invention as set forth in the appended claimsand equivalents thereof.

The methods described herein may be implemented by various means. Forexample, these methods may be implemented in hardware, firmware,software, or a combination thereof. For a hardware implementation,functions may be implemented within one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, electronic devices, other electronicunits designed to perform the functions described herein, a computer, ora combination thereof.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a Digital SignalProcessor (DSP), an Application Specific Integrated Circuit (ASIC), aField Programmable Gate Array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in Random Access Memory (RAM), flashmemory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM),Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, aremovable disk, a CD-ROM, DVD, Blu-Ray disc, or any other form ofstorage medium. An exemplary storage medium is coupled to the processorsuch that the processor can read information from, and write informationto, the storage medium. In the alternative, the storage medium may beintegral to the processor. The processor and the storage medium mayreside in an ASIC. The ASIC may reside in a user terminal. In thealternative, the processor and the storage medium may reside as discretecomponents in a user terminal.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

I claim:
 1. A method to select a receiving apparatus for co-channeloperation, the method comprising: transmitting a first data sequence ata first power level on a first channel to a first remote station;transmitting a second data sequence at a second power level on the firstchannel to the first remote station; receiving a signal indicatingcharacteristics of the first data sequence and the second data sequencefrom the first remote station; and determining whether the first remotestation is a candidate for co-channel operation based on thecharacteristics of the first data sequence and the second data sequence,wherein the co-channel operation is a multi-user on one slot (MUROS)operation.
 2. The method of claim 1, wherein the first data sequence andthe second data sequence are training sequence codes (TSCs).
 3. Themethod of claim 1, wherein the received signal indicates a first biterror probability measured by the first remote station corresponding tothe first data sequence.
 4. The method of claim 3, wherein the first biterror probability is compared to a threshold when determining whetherthe first remote station is a candidate for co-channel operation.
 5. Themethod of claim 3, wherein the received signal also indicates a secondbit error probability measured by the first remote station correspondingto the second data sequence, and wherein the first bit error probabilityand the second bit error probability are used to determine whether thefirst remote station is a candidate for co-channel operation.
 6. Themethod of claim 1, wherein the first remote station is a downlinkadvanced receiver performance (DARP) receiver, wherein the remotestation is determined to be a candidate for co-channel operation, andfurther comprising selecting a second remote station to pair with thefirst remote station on a multi-user on one slot (MUROS) co-transmitchannel (TCH).
 7. The method of claim 6, wherein the second remotestation is a DARP receiver or a non-DARP receiver.
 8. The method ofclaim 6, wherein the second remote station is selected based on at leastone of a power ratio between the first remote station and the secondremote station, codecs that can be applied to both the first remotestation and the second remote station based on conditions, and atraining sequence to be used.
 9. A wireless device configured forselecting a receiving apparatus for co-channel operation, comprising: aprocessor; memory in electronic communication with the processor; andinstructions stored in the memory, the instructions being executable bythe processor to: transmit a first data sequence at a first power levelon a first channel to a first remote station; transmit a second datasequence at a second power level on the first channel to the firstremote station; receive a signal indicating characteristics of the firstdata sequence and the second data sequence from the first remotestation; and determine whether the first remote station is a candidatefor co-channel operation based on the characteristics of the first datasequence and the second data sequence, wherein the co-channel operationis a multi-user on one slot (MUROS) operation.
 10. The wireless deviceof claim 9, wherein the first data sequence and the second data sequenceare training sequence codes (TSCs).
 11. The wireless device of claim 9,wherein the received signal indicates a first bit error probabilitymeasured by the first remote station corresponding to the first datasequence.
 12. The wireless device of claim 11, wherein the first biterror probability is compared to a threshold when determining whetherthe first remote station is a candidate for co-channel operation. 13.The wireless device of claim 11, wherein the received signal alsoindicates a second bit error probability measured by the first remotestation corresponding to the second data sequence, and wherein the firstbit error probability and the second bit error probability are used todetermine whether the first remote station is a candidate for co-channeloperation.
 14. The wireless device of claim 9, wherein the first remotestation is a downlink advanced receiver performance (DARP) receiver,wherein the remote station is determined to be a candidate forco-channel operation, and wherein the instructions are furtherexecutable to select a second remote station to pair with the firstremote station on a multi-user on one slot (MUROS) co-transmit channel(TCH).
 15. The wireless device of claim 14, wherein the second remotestation is a DARP receiver or a non-DARP receiver.
 16. The wirelessdevice of claim 14, wherein the second remote station is selected basedon at least one of a power ratio between the first remote station andthe second remote station, codecs that can be applied to both the firstremote station and the second remote station based on conditions, and atraining sequence to be used.
 17. A computer-program product, thecomputer-program product comprising a non-transitory computer-readablemedium having instructions thereon, the instructions comprising: codefor causing a base station to transmit a first data sequence at a firstpower level on a first channel to a first remote station; code forcausing the base station to transmit a second data sequence at a secondpower level on the first channel to the first remote station; code forcausing the base station to receive a signal indicating characteristicsof the first data sequence and the second data sequence from the firstremote station; and code for causing the base station to determinewhether the first remote station is a candidate for co-channel operationbased on the characteristics of the first data sequence and the seconddata sequence, wherein the co-channel operation is a multi-user on oneslot (MUROS) operation.
 18. The computer-program product of claim 17,wherein the first data sequence and the second data sequence aretraining sequence codes (TSCs).
 19. An apparatus to select a receiverfor co-channel operation, the apparatus comprising: means fortransmitting a first data sequence at a first power level on a firstchannel to a first remote station; means for transmitting a second datasequence at a second power level on the first channel to the firstremote station; means for receiving a signal indicating characteristicsof the first data sequence and the second data sequence from the firstremote station; and means for determining whether the first remotestation is a candidate for co-channel operation based on thecharacteristics of the first data sequence and the second data sequence,wherein the co-channel operation is a multi-user on one slot (MUROS)operation.